A wide variety of gas sensor types are utilized to detect gases and other ambient air conditions. For example, electrochemical sensors are well known. Such sensors may include the use of a metal or plastic can, which houses a liquid electrolyte having electrodes immersed in the liquid. An opening or gas diffusion barrier allows atmosphere to ingress and make contact with a gas-sensing electrode. Infrared sensors are also well known. Infrared sensors advantageously utilize the characteristics of gases which show differing absorption spectrum at various infrared frequencies. Further, metal oxide based gas sensors, such as sensors employing precious metal (Pt, Pd, Au, Ag)-activated SnO2, are also known. Such sensors may utilize porous metal oxides which exhibit a shift in electrical parameters when exposed to differing gases. For example, such electrical parameters may include resistance and capacitance characteristics. Such metal oxide sensors may be housed in a metal and/or plastic cylindrical can or ceramic housing with an opening provided on one end of the can to allow ingress of gas through an active charcoal filter to contact a porous metal oxide bead that is positioned within the can. Often such metal oxide based sensors utilize high operation temperatures, for example as high as 300 to 500 degrees Celsius.
It is known to add Sb2O3 to a gas sensitive SnO2 material to lower resistivity to manageable values at room temperature, and to dope the material with platinum (or palladium, gold or silver) to enhance gas response kinetics and possible sensitivity. It is also known to use thick film air-fireable inks in the electronics industry to make capacitors, resistors, dielectrics and conductors. Thick film air-fireable inks have also been used to make gas sensors from platinised Sb-doped SnO2 materials. Such inks typically include an ink vehicle that itself includes a volatile solvent based on terpineol or butyl carbitol and an ethyl cellulose binder. The purpose of the solvent is to both dissolve the binder and to provide a workable liquid-like form for depositing the oxide material. In conventional methods, the ink vehicle is combined with a base oxide material that has been previously prepared by combining Sb2O3 with SnO2, followed by ball milling, calcining and sieving. Before combination with the ink vehicle, platinum is typically added to the previously prepared base oxide material by droplet deposition in which drops of a liquid solution of a platinum salt are applied to the surface of the base oxide material. During heating after deposition of the ink vehicle/oxide combination, the solvent evaporates at approximately 150° C., leaving behind the binder which acts like a cement in holding the powdered oxide together in a so-called “green state” and providing adhesion to the substrate. The ethyl cellulose binder requires a burn-out temperature of approximately 450° C. during later heating, at which time the metal oxide particles also start fusing together to form a sintered material that is considered to be in the “fired state”. During the burn-out process, the platinum salt decomposes and particles of metallic platinum form on the metal oxide surface.
Thick film inks such as described above are typically deposited either by stencil or by screen printing. The former utilizes a solid metal screen with apertures or holes laser-drilled into it, through which the viscous ink is forced. The ink is deposited typically in one pass, with the screen thickness governing the overall wet print thickness. Screen printing is performed by forcing ink through a metal or plastic mesh with the print pattern required achieved by a combination of closed or open apertures. Screen printing allows more flexibility on printed patterns, but requires multiple prints if thicknesses greater than 20 microns are required. Also, the ink is formulated to deliver a lower viscosity than is the case with stencil inks which are more commonly referred to a ‘pastes’.
Droplet deposition and spin coating are alternative deposition techniques for thick film inks. The former requires very low viscosity probably using a water-based solvent or a sol-gel formulation. Problems with clogging of the dispensing nozzle and settling of the heavy oxide particles leading to sedimentation are reported drawbacks of this technique. If very thin coatings are required (5-10 microns), spin-coating is an option, requiring the use of a photo-sensitive binder to enable patterning of the deposit through a mask. This is then followed by washing away the material not hardened by light exposure.
The use of metal oxide based gas sensor materials in combination with integrated circuit technology to provide an integrated gas sensor has been described in U.S. Pat. Nos. 7,554,134, issued Jun. 30, 2009 to Cummins, and 8,007,167, issued Aug. 30, 2011 to Cummins, both of which are assigned to the present assignee and the disclosures of both of which are expressly incorporated by reference herein in their entirety. As described in U.S. Pat. Nos. 7,554,134 and 8,007,167 a single chip wireless gas sensor may include metal oxide sensing materials combined with a microcontroller, wireless transmit/receive circuitry, and other electrical circuits, all on a single integrated circuit.